^Westbound trailblazer at Illinois 1 near Chrisman, December 2020.
US Route 36 runs east-west across the very middle of Illinois, but at points west of Decatur, it's entirely concurrent with Interstate 72. See the Interstate 72 page for points on US Route 36 west of Decatur. Through Decatur and points east, US Route 36 is a surface road. It mostly connects rural communities, staying south of Interstate 74 and Champaign-Urbana.
US Route 36 enters the state of Illinois on Interstate 72's bridge across the Mississippi River from Hannibal, Missouri...
US Route 36 stays concurrent with Interstate 72 (click for details) through more than half of Illinois's width, then it finally splits away from Interstate 72 to head into downtown Decatur: